This invention relates generally to optical field flatteners.
Optical systems naturally image on curved focal “planes” (focal loci). The human eye makes use of a curved retina that is, to first order, conformal with the image surface formed by the cornea and the lens. There are some well established techniques, discussed below, for flattening the image surface of an optical system, but these techniques use up degrees of freedom in the optical design (i.e., thus increasing optical system complexity) and/or use the addition of a corrector plate near the image surface. In high performance systems such as hyperspectral sensors and ultra-wide field imagers, the field curvature problem is difficult for conventional field flattening technology.
Diffractive optical elements take many forms which may be used as the dispersive elements in hyperspectral sensors. These angularly dispersive elements include, among others, a) reflection and transmission gratings, and b) diffractive elements with optical power such as holographic and binary (lithographically fabricated surface relief) lenses. Use of these and other dispersive elements typically results in strongly curved focal loci, as shown in the extreme example of FIG. 1.
The focal surface of the off-axis holographic lens of FIG. 1 is used to illustrate an extreme case where the dispersion over an ultra-wide band (0.5-2 microns) forms a focal locus that is both strongly curved and extended.
In FIG. 1, an off-axis diffractive lens is used to focus and disperse an incident wave that emanates from a distant axial point. It is apparent the focal locus spanning wavelengths from 0.5 to 2 microns is both curved and extended.
Applicable detector arrays including CCDs and Multiple Quantum Well (MQW) structures, however, are for the most part tightly constrained to planar geometries due to the inherent lithographic and epitaxial fabrication technologies. The strongly curved focal planes are a pronounced mismatch with the planar detector arrays, particularly over large fields. This mismatch is illustrated in FIG. 2, and is the basis of a significant technical problem which evades classic solutions, as described below.
As shown in FIG. 2, the focal surface is strongly curved by the dispersive elements that are used in systems including hyperspectral sensors. Yet solid state detector arrays (e.g., CCD's and MQW device arrays) are inherently planar. This mismatch results in field-dependent defocus, spectral resolution blur, and spectral coordinate distortion that is too large to be corrected by conventional Piazzi-Smyth type optical field flatteners.
The classic approach to compensate for this fundamental mismatch is to make use of refractive solutions in lens design, chiefly the technique originating in 1872 with C. Piazzi-Smyth in which a negative field lens is placed adjacent to the image plane. When a lens is placed near a focal plane it makes little contribution of optical power, but as evidenced by the Petzval Theorem it can have a pronounced effect on the field curvature. This Piazzi-Smyth field flattener is a standard tool used in reducing the mismatch between curved image planes and planar detectors such as the classic photographic plates and solid state detector arrays. While this refractive field flattener approach is effective for the types of field curvatures formed in typical lens systems, it is not capable of correcting the large field curvatures generated in long relay systems or the strong curvatures generated by many dispersive elements in hyperspectral applications as discussed above.
There is therefore a need to provide field flatteners that can correct large field curvatures.